Electron configuration usually refers to the arrangement of electrons around the nucleus of an atom in its ground state, the state in which all of the atom's electrons exist at the lowest possible energy level. The different energy levels occupied by the electrons are often referred to as shells surrounding the nucleus of the atom. Each shell is designated by a whole number, beginning with 1. The higher the number of the shell, the greater its distance from the atom's nucleus. The electrons in each shell exist in regions called orbitals or subshells that are designated s, p, d, and f.

Each electron shell can be occupied by no more than 2n2 electrons, in which "n" stands for the shell number. The first shell, which is closest to the nucleus, will thus contain only two electrons, the second eight, the third 18, and so on. Within a shell, each orbital can be occupied by no more than two electrons.

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Every shell contains the same kind of orbitals found in the previous shell and a new type of orbital as well. The first shell contains only an s orbital, but the second shell contains an s orbital and three p orbitals; each of these p orbitals can hold two electrons, so the combined p orbitals within a shell can hold up to six electrons. The third shell has an s orbital, three p orbitals and five d orbitals. The seven f orbitals first occur in the fourth shell, which also holds an s orbital, three p orbitals, and five d orbitals. Orbitals beyond the f orbitals exist but are rarely discussed.

An electron configuration chart shows the order in which the orbitals within the shells are filled. For example, the electron configuration for the element sodium is 1s2 2s2 2p6 3s1, meaning that sodium's 11 electrons are found in the first, second, and third electron shells. The s orbitals of the first and second shell each contain two electrons, and the p orbital of the second has six electrons. The s orbital of the third shell contains only one electron; its three p orbitals and five d orbitals are unoccupied.

When writing electron configuration notation, the superscript on the letter indicating a type of orbital can never be higher than the maximum number of electrons that can occupy that type of orbital. The superscripts for s, p, d and f will never be higher than 2, 6, 10 and 14, respectively.

Lower energy shells and orbitals are filled before those with a higher energy level. This does not mean, however, that one shell will be completely filled before the electrons begin to occupy the next shell. A configuration chart shows that the 4s orbital will be occupied before the 3d orbitals. This happens because as the number of electrons increases, the electrons interact with one another and create conditions in which the higher orbital is the lowest energy state for the next electron to occupy.

Understanding electron configuration is particularly important to the study of chemistry. This is because chemical reactions generally occur in the valence or outer-shell electrons. The electron configuration of the valence shell provides important information about how each element reacts with the others.

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